The present invention relates to an air brake system actuator, and more particularly to an improved air brake chamber, or service brake actuator, for use with such an air brake system.
Fluid-operated braking systems such as air brake systems have long been used to control the movement of motor vehicles in a safe and effective manner. In particular, air brakes are commonly used on commercial vehicles, such as trucks which typically have large gross vehicle weights. The considerable inertial mass of these heavy-duty vehicles in combination with the high speeds at which they travel require a braking system which responds rapidly with substantial braking power. One system component which is instrumental in the operation of air brake systems is the brake actuator. The brake actuator provides the force necessary when braking a vehicle. Commercial air brake actuators can actuate the normal surface brakes as well as parking/emergency brakes. The brake actuators typically have a network of air chambers defined by one or more diaphragms and/or pistons and a plurality of springs which operate to provide the appropriate braking action in response to inputs by the vehicle driver. In the event an air braking system loses pressure, the brake actuator automatically engages the vehicle brakes.
More specifically, conventional air brake actuators have both a service brake actuator for actuating the brakes under normal driving conditions by the application of compressed air and a spring-type emergency brake actuator which causes actuation of the brakes when air pressure has been released. The emergency brake actuator includes a strong compression spring which forces application of the brake when air is released. This is often referred to as the spring brake.
Typically, the spring brake actuator is disposed in tandem with the service brake actuator. When full pressure is applied to the spring brake actuator, air pressure acting against a diaphragm compresses the compression spring. A spring brake actuator rod is held in a retracted position by a relatively small return spring, thus not affecting the operation of the brake. When the brake is to be applied during normal driving operation, compressed air is provided to the service brake actuator which, acting against a diaphragm, causes a service brake push rod to be extended and causes the brakes to be applied with an application force which is proportional to the air pressure applied to the service brake actuator. In the event of a loss of air pressure or an intentional exhaustion of air from the spring brake actuator, the brake will be mechanically activated by the force of the compression spring acting on the spring brake actuator rod which in turn acts upon the service brake push rod to apply the brakes. Thus, the spring brake portion serves both as a parking brake and an emergency brake.
In a typical prior art air brake system, the spring brake actuator and the service brake actuator are disposed in a single housing comprising a spring brake portion and a service brake portion. The service brake portion includes an air chamber partially defined by a flexible service diaphragm acting against a service brake push rod and a return spring to assure proper release of the brake when air is exhausted from the air chamber. The spring brake portion includes a spring chamber and an air chamber, both partially defined by a spring brake diaphragm acting against a spring pressure plate to compress the compression spring in the spring chamber when air pressure is applied to the spring brake diaphragm in the air chamber. An actuating rod extends through the spring brake diaphragm to integrally connect with the pressure plate. In operation, it is pushed outwardly from the air chamber through a housing opening and bearing provided with a pneumatic seal to engage the service diaphragm and push rod of the service brake, thereby causing the brake to be applied. The spring brake diaphragm is provided with a centrally disposed aperture having an annular edge and the actuator rod extends through the opening and engages the annular edge to form an airtight seal. The actuator rod is hollow with a central bore and a brake releasing caging bolt extends into the central bore. An end plate on the bolt engages the spring brake pressure plate to draw the spring to a compressed state when the spring is caged by rotation of the bolt.
In brake actuators of this type, such as those disclosed in U.S. Pat. Nos. 5,315,918, 5,361,877, 5,937,733 and 6,012,378, the diaphragms are typically flexible, cup-shaped and have an outer radial rim which is received and compressed between flange portions of a housing and cover. Such diaphragms are generally formed of a rubber material with a fibrous reinforcement disposed therein. Diaphragms are often molded in an ovoid shape making it somewhat difficult to align and assemble the diaphragm in the brake actuator housing. The difficulties in assembly can often lead to leakage of the seal established between the diaphragm and the flange portions of the housing and cover. Great care is taken in the manufacture of the brake actuators in order to test for leaking actuator assemblies, identify leaking brake actuators assemblies, and to eliminate leaking brake actuator assemblies. If the brake actuator assembly leaks, the brake actuator may fail.
Moreover, in this type of brake actuator assembly, it is important during assembly that the rim portion of the flexible diaphragm be properly seated and aligned prior to being compressed between the opposed radial flanges of the housing and the cover. During assembly the flexible diaphragm must be coaxially aligned with the radial flanges of the opposed housing members. However, due to the ovoid shape of typical diaphragms as received from the manufacturer and curvature of the surfaces of the opposed radial flanges, alignment of the diaphragm and housing is difficult, thereby increasing the potential for leakage.
Additionally, the thickness of radial rim portion of the diaphragm causes difficulty in the assembly of the brake actuator. The thickness of the rim portion of the diaphragm requires that the housing and cover be placed in compression against the diaphragm during assembly in order to facilitate the application of a band clamp which holds the housing and cover in sealing engagement with the diaphragm.
These problems associated with traditional brake actuator assemblies were attempted to be addressed by U.S. Pat. No. 5,922,297 which discloses a brake actuator assembly which includes a housing having an open end and a generally radial annular flange portion surrounding the open end, a cover having a generally radial annular flange portion generally coaxially aligned with the generally radial annular flange portion of the housing, a cup-shaped flexible diaphragm having an outer radial annular rim portion located between the generally radial annular flange portions of the housing and the cover, and an annular clamp which may be separate or integral with one of the housing members generally surrounding and compressing the generally radial annular flange portions of the housing and cover against the rim portion of the flexible diaphragm. The rim portion of the diaphragm includes a wedge-shaped edge portion and co-axially aligned flats on opposite sides of the rim portion adjacent the wedge-shaped edge portion which aligns the flange portions of the housing and the cover.
While this design may aid in the assembly of the brake actuator, and possibly reduce the frequency of leaks caused by misalignment of the diaphragm during assembly, it still suffers from disadvantages of its own. One of these disadvantages is that, like the prior art described above, the actuator disclosed in U.S. Pat. No. 5,922,297 relies on the axial compression forces between the two parts of the actuator housing (i.e., the flanges on the service housing and the service housing cover), and thus on the force exerted by the clamp band holding the two parts together, in order to hold the diaphragm in place. This is undesirable for a number of reasons, one of which is that the tightness of the chamber (i.e., the forces sustainable by the diaphragm within the chamber) is dependent upon the axial compression forces between the housing parts during assembly. Thus, for example, if it is desirable to increases the forces sustainable by the diaphragm without the diaphragm pulling away from the housing, it is necessary to exert a greater clamping force on the housing during assembly. Such is imprecise and may be difficult to achieve, and is therefore undesirable.
What is desired, therefore, is a service brake actuator which is easy to assemble, which is not prone to leaks, which includes a diaphragm which is readily properly seated, which does not rely on axial compression forces between parts of the actuator housing in order to hold the diaphragm in place, and which allows for the forces sustainable by the diaphragm to be readily varied.
Accordingly, it is an object of the present invention to provide a service brake actuator which is easy to assemble.
Another object of the present invention is to provide a service brake actuator having the above characteristics and which is not prone to leaks.
A further object of the present invention is to provide a service brake actuator having the above characteristics and which includes a diaphragm which is readily properly seated.
Still another object of the present invention is to provide a service brake actuator having the above characteristics and which does not rely on axial compression forces between parts of the actuator housing in order to hold the diaphragm in place.
Yet a further object of the present invention is to provide a service brake actuator having the above characteristics and which allows for the forces sustainable by the diaphragm to be readily varied.
These and other objects of the present invention are achieved by provision of a service brake actuator having a housing base having a center portion and a housing cover having a center portion. The center portion of the housing base and the center portion of the housing cover telescopingly fit together in an assembled position and define an annular space therebetween. A diaphragm having a thickness greater than the thickness of the annular space between the center portion of the housing base and the center portion of the housing cover is held in the annular space by radial forces generated by compression of the diaphragm in the annular space. It should be understood that the magnitude of the radial forces holding the diaphragm in the annular space is variable depending on the thickness of the annular space.
In one embodiment, the center portion of the housing cover telescopingly fits within the center portion of the housing base in an assembled position, and an annular inner surface of the center portion of the housing base and an annular outer surface of the center portion of the housing cover define the annular space therebetween. In this case, the magnitude of the radial forces holding the diaphragm in the annular space is increased by increasing a diameter of the annular outer surface of the center portion of the housing cover, by decreasing a diameter of the annular inner surface of the center portion of the housing base, or by doing both. Similarly, the magnitude of the radial forces holding the diaphragm in the annular space is decreased by decreasing a diameter of the annular outer surface of the center portion of the housing cover, by increasing a diameter of the annular inner surface of the center portion of the housing base, or by doing both.
In another embodiment, the center portion of the housing base telescopingly fits within the center portion of the housing cover in an assembled position, and an annular inner surface of the center portion of the housing cover and an annular outer surface of the center portion of the housing base define the annular space therebetween. In this case, the magnitude of the radial forces holding the diaphragm in the annular space is increased by increasing a diameter of the annular outer surface of the center portion of the housing base, by decreasing a diameter of the annular inner surface of the center portion of the housing cover, or by doing both. Similarly, the magnitude of the radial forces holding the diaphragm in the annular space is decreased by decreasing a diameter of the annular outer surface of the center portion of the housing base, by increasing a diameter of the annular inner surface of the center portion of the housing cover, or by doing both.
Preferably, a clamp ring is provided for holding the housing base and the housing cover together in the assembled position, the clamp ring exerting substantially no compressive axial forces on the diaphragm. In one embodiment, the clamp ring comprises a two-piece clamp ring having two pieces bolted together. Also preferably, the diaphragm includes an outwardly projecting peripheral annular flange located outside of the annular space, the annular flange inhibiting the diaphragm from being pulled through the annular space under forces to which the diaphragm is subject.